1. Field of the Invention
This invention relates in general to a web transport system of the reversing-type and, in particular, to apparatus for determining the speed of a motor associated with such a system.
2. Description Relative to the Prior Art
It is often desirable to know the speed of an electric motor and/or to produce a signal representative of motor speed, thereby to control the operation, say, of a web transport system employing such motor. Although tachometers, such as optical and electromagnetic speed sensors, are well known to the art of motor speed sensing and control, they constitute an expense which, desirably, might otherwise be avoided. To this end, it is also well known that the back electromotive force (emf) of a motor varies in direct proportion to motor speed and, as specifically taught in U.S. Pat. No. 3,733,529, such back emf can be determined by use of a bridge circuit comprising the armature resistance of the motor. Although the operation of a back emf sensing bridge circuit will be discussed in greater detail later, it is noted here that a problem inherent in the use of such a bridge circuit concerns the unsettling of the bridge balance by thermal effects associated with the armature resistance: As the motor runs, its armature heats up--thereby becoming a higher resistance--and causes the bridge circuit to become unbalanced. This causes the bridge circuit to register a wrong measure of motor speed.
To nullify the effect of resistance variation with temperature in a back emf sensing bridge circuit, it has been the practice in the art to situate a resistor, which in the bridge circuit corresponds to or equates with the armature resistance, right on (or at least proximate to) the housing of the motor whose speed is to be sensed. Thus, what thermally affects the armature resistance also thermally affects the corresponding resistor, with attendant relative stability in the balance of the bridge circuit. Such a technique works fine . . . but only to a point, i.e. speed sensing by such a technique can be in error by as much as 5%.